In the processing of denim goods and other fabrics, particularly articles of clothing, a common practice to produce a "stonewashed" or worn effect is to wash the articles with approximately an equal weight of natural or synthetic stones. Commerical washers used to achieve the best results are usually 125 lbs. to 800 lbs. capacity with an open-pocket design which effects maximum abrasion of the article by rolling of the article with the stones. This rolling or tumbling also causes piece-to-piece abrasion of the fabric, resulting in a stonewashed look due to the removal of a portion of the dyes, particularly indigo dyes, in the fabric. The degree of harshness of the abrasion or dye removal is controlled by load size, amount of stones used, water levels, and time of the wash or tumbling cycle.
The use of natural or synthetic stones in stonewashing processes has a number of deleterious effects on the washing equipment used, as well as on the fabrics being treated. Thus, the stones tend to finely abrade the inner metal shell of the washing equipment, rendering the equipment unsuitable for later washing of articles, such as sheets, towels, shirts, etc., which may be physically damaged, reducing the useable life of such articles. A further result of the abrasion process is that a sand or sludge formed by destruction of the stone during the wash process collects in sumps, filters, drainage areas and plumbing of the washing equipment, as well as in the fabric being treated. Another drawback is the propagation of dust generated from handling the stones in both storage and shipment prior to washing.
The most common abrasive material used for stonewashing is pumice stone. Other examples include ordinary rocks, sand, ceramic compositions, pieces of rubber tires, wood, rope, rigid polymeric foam forms, broken glass, etc. All of these materials must be removed from the washing equipment and the fabric after treatment, and many of them tend to disintegrate during the tumbling and abrasion process.